This invention relates to a pressure sensitive adhesive for use on paper products which are subjected to high temperature conditions, and more particularly to an adhesive for use on documents such as mailer forms produced by laser printing.
The use of "self" adhesives for use in mailer forms is well known. In the past, heat seal adhesives were used on forms which were not subjected to heat and pressure until the final printed form was sealed by applying heat to the adhesive. However, new adhesives were required with the advent of laser printers, which generally use heat and pressure (fuser rolls) for adhering toner images to paper. These adhesives need to form strong bonds under pressure alone and be able to withstand fuser roll temperatures of up to 450.degree. F. without becoming tacky.
Much effort has been made to adapt "self" adhesives such as natural rubber latex coatings to the needs of laser printers. These coatings generally are adherent with other coatings of the same material but are not adherent toward other surfaces. The pressure seals made after printing in laser printers using such "self" adhesives have not been entirely satisfactory for several reasons. First, such adhesives have adhered to the fuser rolls, causing fouling or jamming. Second, silicone oils which are commonly applied to fuser roll surfaces have transferred to the adhesives, preventing formation of strong self-adhesive bonds. Third, adhesive coated forms are stored in stacks, packs or rolls prior to printing and "blocking" often occurs, which causes the adjacent piles to stick together, thus preventing proper feeding. Finally, such prior adhesive coatings lose bond-forming ability with age.
Some efforts have been made to correct these problems using adhesives such as the one disclosed in U.S. Pat. No. 4,918,128. This adhesive is composed of a graft polymerized natural rubber latex with styrene and methyl methacrylate comonomers and a non-thermoplastic particulate solid.
Still, there remains a need in the art for a pressure sensitive adhesive which will not become tacky under high temperatures, lose adhesive properties due to contact with silicone oils, "block" when stacked prior to printing, or lose bond-forming ability with age.